1. Field of the Invention
The present invention is broadly concerned with an improved method for the treatment of oil and/or gas well formations using chlorine dioxide. More particularly, the invention pertains to such methods wherein substantial quantities of chlorine dioxide are generated in situ within a well, thereby eliminating the need for complex equipment for the above-ground generation of chlorine dioxide and also permitting use of substantially greater concentrations of chlorine dioxide at the well formation.
2. Description of the Prior Art
It is well known that during the production life of oil and/or gas wells, the well formations tend to become plugged with iron sulfides and sulfates or polymers added to injection water for increasing water viscosity and sweep efficiencies. Such plugging decreases well production. In response to this problem, a well-established technique is to inject chlorine dioxide into the well. Chlorine dioxide rapidly oxidizes the naturally occurring iron compounds, and also is effective at breaking up and removing organic solids and added polymers.
The chemistry of chlorine dioxide precludes its storage, and U.S. Department of Transportation regulations prohibit truck transport of chlorine dioxide solutions. As a consequence, well treatments using chlorine dioxide have heretofore required complex, costly equipment for generation of chlorine dioxide at the well site.
In one commercial technique known as the xe2x80x9cDIKLOR-Sxe2x80x9d process, a chlorine dioxide generator is moved adjacent a well to be treated, along with a water truck and a high pressure pump truck. Sodium chlorite, sodium hypochlorite and hydrochloric acid are added to the generator and water is supplied thereto. An aqueous solution of chlorine dioxide is produced by the generator which is then introduced into the well. The maximum concentration of chlorine dioxide obtainable using this process is about 4500 ppm.
It will be appreciated that the capital costs associated with equipment for the safe generation and handling of chlorine dioxide at a well site is considerable. Moreover, owing to the dangerous nature of chlorine dioxide, skilled, experienced operators must be employed. In fact, these capital and personnel costs, which must of course be passed along to the well owner, have significantly limited the use of chlorine dioxide as a well stimulant. There is accordingly a real and unsatisfied need in the art for an improved process which allows the use of high concentrations of chlorine dioxide down hole for well stimulation purposes, while at the same time substantially minimizing capital and manpower costs.
The present invention overcomes the problems outlined above and provides a new method for the treatment of oil and/or gas wells using chlorine dioxide to stimulate production. Instead of generating chlorine dioxide above ground and then introducing it into the well, the present invention involves the in situ below grade generation of chlorine dioxide. In this way, all of the complex and costly equipment characteristic of the prior art chlorine dioxide well stimulation methods is eliminated. Moreover, because the chlorine dioxide is generated in situ, substantially greater concentrations of chlorine dioxide can be used. Indeed, in preferred forms, the in situ reaction of the invention generates a reaction product having at least about 10,000 ppm, and more preferably at least about 35,000 ppm, and most preferably at least about 90,000 ppm of chlorine dioxide therein.
In preferred forms, an oil and/or gas well having an upright passageway leading to an underground formation is treated, by introducing one or more reactants into the passageway and causing the desired chemical reaction to occur down hole using the one or more reactants so as to generate chlorine dioxide. Normally, chlorite ion and hypochlorite ion are added to the well, along with an acid (e.g., hydrochloric, sulfuric, sulfurous, hydrofluoric, phosphoric, phosphorous and acetic). The ions are provided by appropriate chlorite ion and hypochlorite ion sources, typically alkali metal and alkaline earth chlorites and hypochlorites. The chlorite and hypochlorite ion sources can be added as separate reactants, or can be in the form of a stabilized chlorite/hypochlorite ion mixture.
Virtually all producing wells have a standing column of formation fluid within the annulus thereof. Therefore, the preferred chlorite ion, hypochlorite ion and acid reactants are added atop this standing column. In order to drive the reactants and reaction products down to formation level, a pushing fluid (usually water) is added after the reactants are introduced into the well in sufficient quantity to force these materials down to formation level.
The chlorite ion, hypochlorite ion and acid reactants may be added in serial order or with the intermediate introduction of water therebetween. In any case, these reactants are introduced at sufficient quantities so as to insure the generation of copious amounts of chlorine dioxide within the well.